The characteristics of a thin-film transistor widely vary depending on the material used for the semiconductor layer. For example, when a low-temperature polycrystalline silicon semiconductor is used for the semiconductor layer, the reliability of the thin-film transistor is increased. When an oxide semiconductor is used for the semiconductor layer, the off-current of the thin-film transistor is less. In a display device comprising a thin-film transistor having a semiconductor layer formed of polycrystalline silicon and a thin-film transistor having a semiconductor layer formed of oxide on the same substrate, a technology for providing the semiconductor layer formed of oxide above the semiconductor layer formed of polycrystalline silicon is known. In the display device, cleaning is applied to remove a natural oxide film formed on the surface of the semiconductor layer formed of polycrystalline silicon after a contract hole which penetrates to the semiconductor layer formed of polycrystalline silicon is formed. At this time, the semiconductor layer formed of oxide might be etched by a cleaning liquid such as hydrofluoric acid (HF) used for cleaning.